The Brent Group in the Brent Field contains over 1000x 106bbl of recoverable oil in a simple gently dipping half-graben structure. It is 243 m (800 ft) thick and represents a sequence amplified by synsedimentary fault-block subsidence close to the axis of the Viking Graben. On a regional scale the depositional model is of a major shoreface and coastal sequence, the Rannoch and Etive Formations, underlying the partly time-equivalent shallow lagoonal to emergent delta-plain deposits of the Ness Formation. However, more localized and detailed sedimentary modelling is required to predict reservoir geometries and production behaviour, particularly in the Ness Formation.The Ness Formation is the largest reservoir-bearing unit in the Brent Field, being up to 160 m (525 ft) thick. Five main facies associations form reservoir-quality rock: varied channel sandstones, fluvial sheet sandstones, major mouth bars, lagoonal shoals (which are usually associated with, and capped by, thin beaches) and lagoonal sheet sandstones. These associations make up 50%-60% of the sand-rich sequence and are embedded in the nonreservoir facies associations: lagoonal mudstones, emergent flood-plain mudstones and allochthonous and autochthonous coals. The coals are often field wide in extent. Within the amplified Ness Formation the most important environments were the lagoonal deltas which were wave and fluvial dominated with minor tidal influence. As a result of this importance the sand bodies produced by these deltas are subjected to a more detailed description in this paper. Wave energies were sufficient to redistribute large amounts of sediment and indicate significant fetch in the brackish lagoons diluted by major freshwater input.Sand-body geometries are very variable, reflecting both localized controls on sediment distribution, particularly immediately above the compacting shoreface pile, and more regional effects including coarse-sediment supply, basinal processes and fault-block subsidence. The dominant controls on sedimentation resulted in regular drowning events on the delta plains and had a significant impact by producing a strongly layered reservoir.

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Cormorant Block IF' is one of four, large-scale fault-bounded accumulations which together comprise the Cormorant oilfield. Thefield lies some 150 kms NE of the Shetland Islands, in the East Shetland Basin. Oil is trapped in the Middle Jurassic (Aalenian-Bathonian) Brent Group. Block IV is a separate accumulation in a downthrown area formed by a large synsedimentary Iistric fault, which produced a dome-shaped structure with further internal fault-block compartmentalisation. A concerted data-gathering effort on the accumulation included the coring of 43% of the reservoir section penetrated by wells.The Brent Group in Block IVFts into the "classical" overall regressivehransgressive, wavedominated delta model widely accepted for these sediments. However, the synsedimentary boundary fault has had some influence on the shallow-marine Broom and Tarbert Formations and the coastal-plain Upper Ness sediments, in each case preserving a unique and thicker sequence on the downthrown eastern side. The lowermost Broom Formation includes a shallowmarine, Gilbert-delta type deposit that infilled a topographic feature formed by the fault, and is genetically distinct.from the rest of the Group. The Rannoch and Etive delta-shoreface sediments, the main reservoir interval, appear to have prograded through the area towards the NNE so rapidly that no fault influence was felt. Lateral variability at this level can be explained by the location of distributaries in the upper shoreface, controlling the grade of sediment supply and progradation rates of the entire system. The barrier-attached coastal-plain sequence of the lower Ness is chronostratigraphically equivalent to the Etive. and thins rapidly towards the north. The upper Ness is strongly layered by repeated periods of emergent coastal-plain development separated by at least block-wide lagoonal drowning events. Erosion at the base of the shallowmarine Tarbert Formation, and the marked variability of sediment type and thickness in this horizon, suggests that it ,formed over a considerable time-period at the end of Brent Group deposition, as the structure of the block was still developing.

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